Fifteen protesters were arrested and more than 30 people were hurt during overnight clashes with police following the jailing of a Spanish rapper over tweets insulting police and the monarchy, officials said.
Hours after police stormed a university campus in the Catalan city of Lerida to arrest rapper Pablo Hasel who had barricaded himself inside to avoid prison, protesters hit the streets in towns and cities in the northeastern region, police said.
In Barcelona, some 1,700 people massed in a central square, with a handful of troublemakers setting fire to large rubbish bins and hurling stones and other objects at police, an AFP correspondent said.
Similar scenes were repeated in Lerida, the rapper's hometown, while in Vic about 70 kilometres (45 miles) north of Barcelona, protesters attacked a police station, injuring 11 officers, a police spokeswoman said.
Police said a total of 15 people were arrested, while the emergency services said 33 people were lightly injured in the clashes, including 19 police officers.
Hasel missed a deadline last Friday to give himself up to police and begin serving a nine-month jail term handed down in 2018.
At issue was a series of tweets calling former king Juan Carlos I a mafia boss and accusing police of torturing and killing demonstrators and migrants.
Hasel is known for his hard-left views, but his case has become a cause celebre among campaigners who say that prosecuting him is a dangerous assault on free speech.
His sentence caused outrage in Spain and led the government to announce it would make free speech laws less restrictive.